X86: Implement lowest priority interrupts more correctly.

Lowest priority interrupts are now delivered based on a rotating offset into
the list of potential recipients. There could be parasitic cases were a
processor gets picked on and ends up at that rotating offset all the time, but
it's much more likely that the group will stay consistent and the pain will be
distributed evenly.

2 files changed, 19 insertions, 3 deletions

diff --git a/src/dev/x86/i82094aa.cc b/src/dev/x86/i82094aa.cc
index ad9b29dd0..ed936d0cb 100644
--- a/src/dev/x86/i82094aa.cc
+++ b/src/dev/x86/i82094aa.cc
@@ -38,7 +38,7 @@
 
 X86ISA::I82094AA::I82094AA(Params *p) : PioDevice(p), IntDev(this),
     latency(p->pio_latency), pioAddr(p->pio_addr),
-    extIntPic(p->external_int_pic)
+    extIntPic(p->external_int_pic), lowestPriorityOffset(0)
 {
     // This assumes there's only one I/O APIC in the system
     initialApicId = id = p->apic_id;
@@ -189,8 +189,22 @@ X86ISA::I82094AA::signalInterrupt(int line)
                     apics.push_back(localApicIt->first);
                 }
             }
-            if (message.deliveryMode == DeliveryMode::LowestPriority) {
-                panic("Lowest priority delivery mode is not implemented.\n");
+            if (message.deliveryMode == DeliveryMode::LowestPriority &&
+                    apics.size()) {
+                // The manual seems to suggest that the chipset just does
+                // something reasonable for these instead of actually using
+                // state from the local APIC. We'll just rotate an offset
+                // through the set of APICs selected above.
+                uint64_t modOffset = lowestPriorityOffset % apics.size();
+                lowestPriorityOffset++;
+                ApicList::iterator apicIt = apics.begin();
+                while (modOffset--) {
+                    apicIt++;
+                    assert(apicIt != apics.end());
+                }
+                int selected = *apicIt;
+                apics.clear();
+                apics.push_back(selected);
             }
         }
         intPort->sendMessage(apics, message,
diff --git a/src/dev/x86/i82094aa.hh b/src/dev/x86/i82094aa.hh
index e81d85fa9..c3a832aa9 100644
--- a/src/dev/x86/i82094aa.hh
+++ b/src/dev/x86/i82094aa.hh
@@ -77,6 +77,8 @@ class I82094AA : public PioDevice, public IntDev
     uint8_t id;
     uint8_t arbId;
 
+    uint64_t lowestPriorityOffset;
+
     static const uint8_t TableSize = 24;
     // This implementation is based on version 0x11, but 0x14 avoids having
     // to deal with the arbitration and APIC bus guck.